The present invention pertains to composite wires reinforced with substantially continuous ceramic oxide fibers within an aluminum matrix and cables incorporating such wires.
Metal matrix composites (MMC""s) have long been recognized as promising materials due to their combination of high strength and stiffness combined with low weight. MMC""s typically include a metal matrix reinforced with fibers. In selection of the fiber, it is widely acknowledged that one desires reinforcement fibers possessing high strength, a high elastic modulus, and a low coefficient of thermal expansion.
The use of metal matrix composites in the form of wires as a reinforcing member in bare overhead power transmission cables is of particular interest. The need for new materials in such cables is driven by the need to increase the power transfer capacity of existing transmission infrastructure due to load growth and changes in power flow due to deregulation. Desired performance requirements for such new materials include corrosion resistance, environmental endurance (e.g., UV and moisture), retention of strength at elevated temperatures, and creep resistance.
Important properties for performance are elastic modulus, density, coefficient of thermal expansion, conductivity, and strength. These properties are typically governed by the choice and purity of constituents (i.e., material of the metal matrix and the fiber content) in combination with the fiber volume fraction. Of these properties, emphasis has been placed on the development of wires made from fibers with high tensile strength and stiffness. The focus on producing materials of high strength is driven in part by the assumption that in order for the composite to compete economically with conventional materials such as steel, its strength should be as high as possible. For example, in Ozawa et al., xe2x80x9cMechanical Properties of Composite Conductors using SiC Fiber Resinforced Aluminum Composite Wires,xe2x80x9d The Electricity Society National Symposium, 1996, which discloses an aluminum wire reinforced with high strength fiber marketed under the trade designation xe2x80x9cNICALON,xe2x80x9d the need for MMC wires of xe2x80x9chigh strengthxe2x80x9d for use in overhead power transmission cables is described.
The present invention-relates to substantially continuous fiber aluminum matrix composites. Embodiments of the present invention pertain to aluminum matrix composite articles, preferably elongated metal composite articles such as wires, tapes, etc. Such articles preferably include a plurality of substantially continuous, longitudinally positioned fibers contained within a matrix that includes aluminum (e.g., high purity aluminum or alloys thereof). Preferably, the matrix of the wire includes at least 99.95 percent by weight aluminum, based on the total weight of the matrix.
The aluminum matrix composites of the present invention are formed into wires exhibiting desirable strength-to-weight and thermal expansion characteristics, high electrical conductivity, and low modulus. Such wires are well-suited for use as core materials in power transmission cables, as they provide electrical and physical characteristics which offer improvements over power transmission cables known in the prior art.
The materials of the present invention are advantageous for wires and cables because they provide less sag when heated due to the low coefficient of thermal expansion. Additionally, compared with steel wires or composite wires reinforced with high modulus materials, the wires of the present invention are capable of reducing the tension on supporting towers when the cables are exposed to high mechanical loads (such as combined ice and wind load) due to their low modulus.
In one embodiment, the present invention provides an aluminum matrix composite article that includes a plurality of fibers in a matrix including aluminum. In this embodiment, the fibers include, on a theoretical oxide basis, Al2O3 in a range of about 35 weight percent to about 75 weight percent, SiO2 in a range of greater than zero weight percent to less than about 50 weight percent, and B2O3 in a range of greater than about 5 weight percent, based on the total metal oxide content of the respective fiber. In this embodiment the wire has a nonlinear coefficient of thermal expansion over a temperature of xe2x88x9275xc2x0 C. to 500xc2x0 C., a modulus of no greater than about 105 GPa (15 Msi), and an average tensile strength of at least about 350 MPa (50 ksi).
In another embodiment, the present invention provides an aluminum matrix composite wire that includes a plurality of substantially continuous, longitudinally positioned fibers in a matrix including aluminum. In this embodiment, the fibers include, on a theoretical oxide basis, Al2O3 in a range of about 35 weight percent to about 75 weight percent, SiO2 in a range of greater than zero weight percent to less than about 50 weight percent, and B2O3 in a range of greater than about 5 weight percent, based on the total metal oxide content of the respective fiber. In this embodiment the wire has a nonlinear coefficient of thermal expansion over a temperature of xe2x88x9275xc2x0 C. to 500xc2x0 C., a modulus of no greater than about 105 GPa (15 Msi), and an average tensile strength of at least about 350 MPa (50 ksi).
In another embodiment, the present invention provides a method for making an aluminum matrix composite wire that includes a plurality of substantially continuous, longitudinally positioned fibers in a matrix that includes aluminum. The method includes: providing a contained volume of molten matrix material; immersing a plurality of substantially continuous fibers into the contained volume of molten matrix material wherein the fibers comprise, on a theoretical oxide basis, Al2O3 in a range of about 35 weight percent to about 75 weight percent, SiO2 in a range of greater than zero weight percent to less than about 50 weight percent, and B2O3 in a range of greater than about 5 weight percent, based on the total metal oxide content of the respective fiber; imparting ultrasonic energy to cause vibration of at least a portion of the contained volume of molten matrix material to permit at least a portion of the molten matrix material to infiltrate into and wet the plurality of fibers such that an infiltrated, wetted plurality of fibers is provided; and withdrawing the infiltrated, wetted plurality of fibers from the contained volume of molten matrix material under conditions which permit the molten matrix material to solidify to provide an aluminum matrix composite wire comprising a plurality of the fibers, wherein the fibers are substantially continuous, longitudinally positioned in a matrix including aluminum, and wherein the wire has a nonlinear coefficient of thermal expansion over a temperature of xe2x88x9275xc2x0 C. to 500xc2x0 C., a modulus of no greater than about 105 GPa, and an average tensile strength of at least about 350 MPa.
In another embodiment, the present invention provides a cable that includes at least one aluminum matrix composite wire that includes a plurality of substantially continuous, longitudinally positioned fibers in a matrix including aluminum. In this embodiment, the fibers include, on a theoretical oxide basis, Al2O3 in a range of about 35 weight percent to about 75 weight percent, SiO2 in a range of greater than zero weight percent to less than about 50 weight percent, and B2O3 in an amount of greater than about weight 5 percent, based on the total metal oxide content of the respective fiber. Furthermore, in this embodiment, the wire has a nonlinear coefficient of thermal expansion over a temperature of xe2x88x9275xc2x0 C. to 500xc2x0 C., a modulus of no greater than about 105 GPa, and an average tensile strength of at least about 350 MPa.
In yet another embodiment, the present invention provides an aluminum matrix composite wire that includes a plurality of substantially continuous, longitudinally positioned ceramic oxide fibers in a matrix including aluminum. In this embodiment, the ceramic oxide fibers have a modulus of no greater than about 173 GPa (25 Msi), and the wire has a modulus of no greater than about 105 GPa.
In a further embodiment, the present invention provides a cable that includes at least-one aluminum matrix composite wire that includes a plurality of substantially continuous, longitudinally positioned ceramic oxide fibers in a matrix comprising aluminum. In this embodiment, the fibers have a modulus of no greater than about 240 GPa (35 Msi), and the wire has a modulus of no greater than about 105 GPa and an average tensile strength of at least about 350 MPa.
As used herein, the following terms are defined as:
xe2x80x9cSubstantially continuous fiberxe2x80x9d means a fiber having a length that is relatively infinite when compared to the average effective fiber diameter. Typically, this means that the fiber has an aspect ratio (i.e., ratio of the length of the fiber to the average effective diameter of the fiber) of at least about 1xc3x97105, preferably, at least about 1xc3x97106, and more preferably, at least about 1xc3x97107. Typically, such fibers have a length on the order of at least about 50 meters, and may even have lengths on the order of kilometers or more.
xe2x80x9cAverage effective diameterxe2x80x9d means the true diameter of a circle having the same cross-sectional area as the measured object. For example, a circular fiber or wire has a measurable diameter from which the area may be accurately computed as xcfx80d2/4. Non-circular, elliptical, oval, or irregular shaped fibers or wires do not have a single measurable diameter, although the cross-sectional area may be measured, and then a diameter computed that would create a circle of the same cross-sectional area. This diameter of the circle is termed the effective diameter.
The average effective diameter is the numerical average of several computed effective diameters.
xe2x80x9cLongitudinally positionedxe2x80x9d means that the fibers are oriented in the same direction as the length of the wire.
xe2x80x9cCoefficient of thermal expansion (CTE)xe2x80x9d means the rate of change of thermal expansion over a specified temperature range measured in the longitudinal direction of the fiber and/or wire. That is:
thermal expansion coefficient=(change in dimensions)/(change in temperature). The instantaneous CTE value is then the slope of the thermal expansion vs. temperature graph calculated at a specific temperature (i.e., the derivative of the equation of the curve plotting thermal expansion versus temperature).
xe2x80x9cNonlinear CTE over a temperature of xe2x88x9275xc2x0 C. to 500xc2x0 C.xe2x80x9d means that a second order curve fit of segments between inflections on a graph of the percent thermal expansion vs. temperature within the specified temperature range shows alternating positive and negative slopes.
xe2x80x9cAverage tensile strengthxe2x80x9d means the numerical average of the measured tensile strengths of several fiber, wire, or cable samples.
xe2x80x9cLongitudinal tensile strengthxe2x80x9d means the stress at which the fiber, wire, or cable fails when tested in the direction of the major axis of the fiber, wire, or cable. This is also equal to the maximum stress applied to the sample. The stress, S, is computed as S=L/A, where L is the maximum load measured during the tensile test and A is the cross-sectional area of the sample prior to testing.
xe2x80x9cModulusxe2x80x9d means the longitudinal tensile modulus. It is the tensile stiffness of the fiber, wire, or cable as measured in the direction of the major axis of the sample. It represents the average stress per unit strain for the sample measured over a given strain increment or strain range. For the wire, modulus is measured between 0 and 0.05% strain.
xe2x80x9cCable modulusxe2x80x9d means the elastic tensile modulus of the cable. The cable modulus is obtained by loading and unloading a cable using tensile testing apparatus to obtain a load-unload deformation curve. The cable is loaded sufficiently so that the constructional stretch of the cable has been taken up and the cable is elastically deformed. The data from the unload region of the curve is used to calculate the cable modulus. This is further described in xe2x80x9cTesting of Wire Ropexe2x80x9d in Theory of Wire Rope, Chapter 6, George A. Costello, Springer-Verlag (1997). The cable modulus can be calculated from measured load-displacement data using the following equation:
E=xcex94F/Ae 
where
E is the calculated cable modulus
xcex94F is change in measured load in the measurement region
A is the total cross-sectional area of the wire in the cable determined prior to testing
e is the change in the measured elongation of the cable in the measurement region specifically,
e=(lf=I0)/l0 
where
l0 is the initial length of the cable in the measurement region
lf is the final length of the cable in the measurement region
xe2x80x9cAverage strain to failurexe2x80x9d means the tensile strain to failure and is the numerical average of the measured strain to failure for several samples. The strain to failure is the elongation or extension of the sample per unit length. It can be represented as:
e=(lfxe2x88x92l0)/l0 
where
e is the elongation or extension of the sample per unit length;
lf is the final gauge length of the sample; and
l0 is the initial gauge length of the sample.
xe2x80x9cTheoretical fiber strain to failurexe2x80x9d is the strain to failure of the fiber calculated using the average fiber tensile strength and is defined by the relationship:
stress=modulusxc3x97strain 
Thus strain=stress/modulus. For fibers available under the trade designation xe2x80x9cNEXTEL 312,xe2x80x9d the measured average tensile stress was 1.68 GPa (244 ksi) and the modulus is reported as 151 GPa (22 Msi). Therefore the strain is 1.1% and the theoretical fiber strain to failure is 1.1%.